Two steps leaching process for recovery of rare earths from moroccan phosphogypsum

Abstract

Phosphogypsum (PG), a primary by-product of the wet process phosphoric acid production, poses significant challenges to the fertilizer industry due to its voluminous generation; for each ton of phosphoric acid (P2O5), 5 tons of phosphogypsum are generated. Traditional management practices involving landfill storage or sea discharge are associated with environmental and health risks. As a result, there is an urgent need for innovative management strategies. One promising avenue is the valorization of valuable elements present in PG, notably rare earths elements (REEs). Although previous research has delved into REEs leaching from PG, fully understanding the mechanisms remains a complex endeavor due to the intricacy of the matrix and the variability in impurities concentrations, including that of rare earths. To address this knowledge gap, this study presents a robust two-step hydrometallurgical process that explores the relationship between REEs concentration and their leaching efficiency. Initial experiments focused on optimizing REEs leaching from Moroccan PG, using three agents: nitric acid, hydrochloric acid, and sulfuric acid, under systematic conditions. Subsequent experiments investigated the correlation between REEs concentrations and their leaching efficiency from PG. Three PG samples with deliberate REEs concentration were prepared and subjected to leaching under previously determined optimal conditions. Both nitric and hydrochloric acids demonstrated high leaching efficiencies with results reaching up to 60%. Intriguingly, when the concentration of REEs in PG was adjusted, the leaching efficiency surged to 76% under analogous conditions. This underlines a strong correlation between REEs leaching efficiency and their concentration in PG, with calcium sulfate solubility, emerging as a pivotal factor influencing both variables. Data from these systematic leaching experiments were further utilized for kinetics investigations, revealing that the diffusion of REEs through the product layer dominates as the rate-controlling step. In conclusion, this research bridges critical gaps in understanding the correlation between the concentration of REEs and their leaching from PG and offers insights propelling REEs recovery for potential widespread applications.